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Old December 3rd, 2009, 06:50 PM
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Brad Harris
1994 d90 300tdi "Nola"
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new tire and wheel backspacing calculation...

new tire and wheel backspacing calculation...
Iím trying to make sure Iím not missing anything here so this is more to check my assumptions than to ask what to doÖ

Iíve been running 255/85/16 tires for a while now but always wanted to do something differentÖ over the last two weeks Iíve been able to get to Moab twice and lost a bead on the tires from 15 lbs on both occasions and then put a 4Ē slice in my sidewallÖ so perfect time to change Iíd say

Iíve decided to go 35x12.5 as a compromise for a daily driver and the type of wheeling I see myself doing (though Iíll probably wish Iíd gone 37 before too long LOL) and Iím trying not to end up with a larger turning radius so:

Iím going taller and wider both of which will cause rubbing sooner while turning fixing the width is obvious but I had to make some assumptions Iíd like some feedback on in trying to take the new height into account:

Actual questions:

question 1: there is a line perpendicular from the front axle that the wheel will rub against at full turn, the location along this line will change depending on the wheel height and width but still along this lineÖ is this correct?

question 2: although the actual pivot point for the turning of the wheel is not directly in line with the edge of the wheel I have not taken this fact into accountÖ how much is this going to throw me off?

question 3: I have ignored extra tire width caused by bulgingÖ how much should I worry about this and how much would I add to account for it?

question 4: I determined my turning angle by the following formula:
=90-(ACOS((C-backspacing+tire hangover)/(tire radius/2)))

Where C= the distance from the line perpendicular to the axle where the rubbing occurs to the end of the hub I donít know this distance but itís only relevant if I want actual numbers not just comparisons

Are there problems with the formula and assumption regarding the constant ďcĒ?

question 5: are there any other issues Iím creating by increasing my backspacing (yea I know increased pressure on joints etc. but I think weíve already determined that on a Defender thatís pretty close to the bottom of the list of the things I need to worry about)

Results: (using 18.5" for "C")
tire size	wheel width	backspacing	additional outside	 turn angle
255/85 r16	7		5.65		0			42.6573123
315/75 r16	7		5.65		1.25			35.24983632
315/75 r16	8		4.5		2.9			42.1774186
315/75 r16	10		4.5		3.9			46.76676519
315/75 r16	8		4.125		3.275			43.8567042
315/75 r16	10		4.125		4.275			48.59037789
315/75 r16	8		3.5		3.9			46.76676519
315/75 r16	8		2.5		4.9			51.7867893
315/75 r16	10		2.5		5.9			57.44306816
315/75 r16	8		4		3.4			44.427004
315/75 r16	8		3.75		3.65			45.5846914
315/75 r16	8		4.25		3.15			43.29180759
35x14.5 r16	10		4.125		5.275			43.8567042
35x14.5 r16	8		4.125		4.275			39.47298186
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